Summary:
New shellcheck incorrectly flags unused functions. Per documentation:
  ShellCheck is currently bad at figuring out functions that are invoked via trap.
  In such cases, please ignore the message with a directive.

MacOS brew formula always installs latest version of shellcheck, so any new development environment
on Darwin will hit this.

Original commit: fd59a42cc49abed0c40d00c13185f1cc3ba465d3 / D46126

Test Plan:
Jenkins: compile only
With shellcheck version 0.11.0: yb_build.sh --skip-build --shellcheck

Reviewers: jharveysmith

Reviewed By: jharveysmith

Subscribers: devops

Differential Revision: https://phorge.dev.yugabyte.com/D46136